Light travels through a vacuum at a bit less than 300,000 kilometres per second.
The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
Light travels through other media at lower speeds.
A vacuum is a region of space that contains no matter.
Matter is anything that has mass and occupies space by having volume.
When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
In other contexts, empty space is not considered to be a medium because it does not contain matter.
When light is described in terms of photons rather than waves the following points are important: